The present invention relates broadly to saws and more particularly to circular saws and milling cutters having ultrahard, wear-resistant teeth, principally used for cutting very hard and abrasive material.
Background: Carbide-Toothed Circular Saws
Cutting tools (especially woodworking tools) often use inserted teeth of a material which is harder than steel. The most common material used for this is a “cemented carbide,” which typically includes small grains of tungsten carbide embedded in a matrix of a high-temperature metal (typically cobalt). Typically the main part of the saw blade is a steel plate, and the carbide teeth are brazed onto the leading edge of tooth profiles which are cut out from the steel plate.
This process limits the minimum spacing between adjacent teeth, and hence limits the number of teeth for a given blade size. For example, carbide-tipped ten-inch blades are currently available with up to 100 teeth, but no more. This implies that the pitch of a conventional carbide-tipped saw cannot be less than one centimeter.
More recently the same technology has been applied to use polycrystalline diamond inserts. However, it appears that the same limitations on tooth pitch will still apply.
Background: Grit-Surfaced (Non-Toothed) “Saws”
A common type of cutting tool is a circular blade which does not have shaped teeth at its edge, but which is simply coated with a diamond grit. Such cutting tools are commonly referred to as diamond “saws,” but in fact they do not perform the same type of material-removal action as is performed by a saw with shaped teeth. A saw with shaped teeth, when it is operating correctly, will carve off chips of material. By contrast, a grit-coated blade will have more of a scraping action. (See generally Jim Effner, Chisels on a wheel (1992); and Peter Koch, Utilization of Hardwoods Growing on Southern Pine Sites (1985); both of which are hereby incorporated by reference.) A cutting action is greatly preferable for many applications, to produce a cleaner cut, lower temperature, and lower power requirements.
Background: Cutting Thin Materials
Sawing action is smoothest when there is always at least one tooth in the cut. For cutting very thin materials, this requires a very fine tooth pitch. If the tooth pitch cannot be made as small as the material thickness, it should still be made as small as possible.
For example, for cutting aluminum extrusions with ten-inch blades, blades with up to 300 teeth are often used; however, these are steel blades, and become dull relatively quickly.
Background: Circular Saws with Tooth Segments
Large circular saws (e.g. 20 to 60 inches in diameter) have long been designed with removable teeth. In a sawmill environment, this is very advantageous, because a damaged tooth can be replaced or resharpened quickly and easily without dismounting the entire blade. A variation of this, as described for example in U.S. Pat. No. 3,633,637, is to use removable multi-tooth segments.
Background: Hard Materials as Applied to Circular Cutting Tools
Since the beginning of the Bronze Age, toolmakers have sought to improve the durability and functionality of tools by modifying their cutting edges. Early processes included work hardening of bronze and adding steel edges to iron implements. The process continues to this day as new, super-hard materials are developed and new applications are found for older ones.
The advantages of extremely hard cutting edges supported by a tough shock-resistant steel band or disk have been recognized for many years. Solutions evolved as described by Neibl, U.S. Pat. No. 907,167, and by Blum, U.S. Pat. No. 1,535,096, both of which are hereby incorporated by reference, where a band of very hard steel was welded to another supporting band of softer steel and teeth were then cut into the hard steel band. Whitaker, U.S. Pat. No. 1,130,649, and Napier, U.S. Pat. No. 1,352,140, both of which are hereby incorporated by reference, approach the same problem by applying special heat treatment and tempering to the cutting edge.
Background: Tungsten Carbide Applications
Following the development of tungsten carbide materials produced by powdered metal technology as described by Schroeter, U.S. Pat. No. 1,594,615, which is hereby incorporated by reference, tungsten carbide moved steadily into tooling applications. For examples of fastening carbide or hard steel into the cutting edges of steel saws see Wilkie, U.S. Pat. No. 2,318,549, and Kolesh, U.S. Pat. No. 2,880,769, both of which are hereby incorporated by reference.
At present, it is common industry practice to tip circular cutting tools with materials that are harder than the hardest of steels. A common material employed is a “cemented carbide,” which typically includes small grains of tungsten carbide bonded into a matrix at high temperatures and pressures by another metal (typically cobalt). Because both the strength and hardness are derived from the compound of tungsten and carbon (WC), and another material (frequently cobalt) serves merely as a binder, the material is commonly referred to as “cemented carbide” which, as a matter of convenience, will hereafter be referred to simply as “carbide.” Carbide saw tips have a hardness of about 92 on the Rockwell A hardness scale.
Some firms manufacture only the steel bodies of milling cutters and circular saws, which are in turn sold to other manufacturers who specialize in applying carbide cutting edges. These bodies, which are normally made of high-carbon alloy tool steel, are hardened, tempered and finished in every way except for tipping. Some tools, principally large milling cutters, may use a clamping mechanism by which carbide cutters are incorporated as replaceable “inserts.” An alternative and more common method of incorporating carbide cutting tips into milling cutters and circular saws is by brazing them directly into pockets ground into the periphery of the cutting tool. The tips may be ¼ to ½ inches long, 0.062 to 0.093 inches thick, and from 0.10 to 0.375 inches wide, depending on the width of the finished tool.
Other manufacturers offer complete saws and milling cutters with carbide tips installed. In either case, the same standard carbide tips are used in the fabrication of the blades. The normal industry practice is to affix the unsharpened carbide tips to the steel bodies by means of brazing, typically with silver bearing brazing material. The tooth geometry is then ground into the carbide by special tools designed for that purpose. FIG. 2 shows a section of a typical configuration of a Carbide Tipped circular saw.
Background: Carbide Tip Failure Mode
Carbide tips installed in the configuration shown by FIG. 1 are fabricated and sharpened by grinding the top, sides, and face of the carbide tip. Likewise, when the carbide tips are worn in use, the tips are generally resharpened by grinding the face and the tip of the carbide cutter. The end of a tool's useful life occurs when, after repeated sharpening of the carbide tip, the face is ground away to the point that it becomes too thin for further sharpening. It is undesirable to grind more off the top of a carbide tip than is absolutely necessary because the effective cutting radius of the tool would be reduced with each regrind. The cutting radius is important because machines that employ cutting tools generally have only a small amount of adjustment to compensate for a change in tool diameter. In addition, many applications permit only a limited tolerance of cutting tool diameter.
Background: Ultrahard Tool Materials as Applied to Circular Cutting Tools
Recently, man-made ultrahard crystalline and polycrystalline compounds have become available, making possible tremendous advances in cutting tool design. Thus, it is now possible to machine substances that have previously been extremely difficult to fabricate. The most common ultrahard materials used in these tools are polycrystalline diamond (PCD), which is 3.6 times harder than tungsten carbide, and cubic boron nitride (CBN), which is 2.8 times harder than carbide.
In practice, thin layers (around 0.5 mm to 1.5 mm thick) of PCD or CBN are bonded to a tungsten carbide substrate. This tungsten carbide substrate typically is in the form of a wafer having a one to one ratio with the PCD or CBN for a combined thickness of around 1.5 to 3.5 mm and a diameter of around 25 mm. Using sophisticated computer controlled tools, these disks are sliced into small pieces which are dimensionally similar to standard carbide blanks and inserts. These pieces are typically brazed or otherwise incorporated onto tool steel bodies in the same manner as carbide tips and inserts. After incorporation onto the tool steel bodies, these ultrahard tips are sharpened by various special techniques.
Background: Ultrahard Tool Braze Configuration
In most applications, the same braze configuration as that used for carbide tools is adequate to support ultrahard tools, the lower strength of the braze material notwithstanding. However, there is a very important and not generally recognized penalty for the use of a “carbide type” brazing configuration, which well be discussed in detail later. FIG. 3 shows a section of a typical cutting tool (in this case a saw) using ultrahard materials. The brazing is facilitated by the carbide substrate which makes the attachment of ultrahard cutting tips very similar to that of attaching standard carbide tips to a steel saw or milling cutter body. However, there are other considerations which need to be taken into account. If the ultrahard tips, specifically those of PCD, are overheated during the brazing process, they revert to a softer form of carbon (graphite), thus rendering the tool useless. This limitation on temperature requires a brazing alloy whose strength is less than optimum for the joint. The ability of the tool body to retain the tip and prevent it from being torn off during operation depends on the surface of the brazed joint and the manner in which it is mounted.
Background: Performance of Ultrahard Materials
The ultrahard materials have truly amazing performance in their proper applications. A PCD tool cutting wooden particleboard may outlast 100 carbide tools of identical construction. However, PCD cannot cut ferrous metals because of a rapid chemical reaction between diamond and iron causing the diamond to revert to graphite. CBN, on the other hand, is capable of machining most materials that PCD cannot. These material include hardened steels, cast iron, and many superalloys. Both PCD and CBN tools are being used in a rapidly expanding number of applications, particularly in the automotive and woodworking industries.
Background: Cost Considerations for Ultrahard Materials
Despite their extraordinary performance, the application of these ultrahard materials is frequently limited by their high cost, which is at least ten times that of tungsten carbide. In addition, because of their extreme hardness, they can only be shaped with great difficulty. PCD can only be ground by special diamond grinding wheels that are no harder than the PCD, and therefore, have a short service life. Other means of shaping PCD include electrodischarge machining (EDM) by either wire or shaped carbon electrode methods. Both of these methods require expensive, specialized computer controlled equipment that further adds to the cost of the tools in which they are incorporated.
Background: Differences Between Ultrahard Materials and Carbide Materials
There are other very important differences in the properties of the ultrahard materials and of the carbide materials that it replaces. PCD tools, unlike carbide tools of identical form can neither be fabricated nor sharpened by grinding the face of the cutting tip. The face of the tip is simply too hard to grind over the area required. Even if the face could be ground, repeated grindings would soon cut away the thin layer of diamond leaving only the carbide substrate. Although the face of a CBN cutting tip can be ground using diamond-grinding wheels, the layer of CBN is even thinner than the diamond. This means that the only practical method for sharpening and servicing rotating tools with PCD and CBN cutters is to lightly grind or otherwise shape the top of the cutting tooth. However, as mentioned before, sharpening the top reduces the cutting diameter of the cutting tool.
Not only is tungsten carbide relatively hard, it is tough. Its failure mode is through abrasive wear. PCD and CBN on the other hand are extremely brittle. The failure mode for these ultrahard materials is generally through microchipping of the cutting edge. To minimize chipping, these ultrahard materials must be rigidly mounted with a minimum of overhang. The typical carbide type tool mounting is less than optimum and, in many cases, unsatisfactory for ultrahard cutting tips because of the lack of support of the entire tip and excessive overhang. This can be seen by referring to FIGS. 2 and 3 where the cutting tip extends out past the extent of the carrier blade. In addition, since ultrahard materials generally become “dull” because of microchipping of their cutting edges, generally, very little material removal during sharpening is required. If the tooth is chipped or broken, it is replaced.
The thickness of the carbide and the size of the brazed area provided by the pocket has an important bearing on the integrity of the joint. However, the same factors that favor a strong brazed joint, also require that a larger amount of very expensive PCD or CBN be used than would be required in a better embodiment for this material.
Method of Fabricating Circular Saw Blades with Cutting Teeth Composed of Ultrahard Tool Material
The present application discloses a cutting tool having a cutting tip composed of two layers. One layer is constructed of an ultrahard material; the other layer is constructed of a non-ultrahard material. The principal axis of the cutting tip is oriented so that the surface of the ultrahard material is almost perpendicular to a radius of the carrier blade rather than parallel to the radius as is the case conventionally.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages: The cutting force is directed principally along the axis of the principal axis (which is parallel to a tangent line at the edge of the carrier blade) of the cutter tip, thus reducing the stress on the braze joint and adding to the rigidity of the cutter tip. Because of the reduced stress in the braze joint, the cutter tools can be designed with a smaller joint area. This has a profound effect on the cost of a cutting tool utilizing ultrahard materials since their cost is directly proportional to the surface area of the material used. The cutting tip can be sharpened and fabricated on conventional machinery since it is perfectly feasible to grind the face 180 in FIGS. 1-4 of the tool and there is no need to grind the top 170 in FIGS. 1-4 of the cutter. Therefore no new machinery is needed to construct or sharpen the cutting tool of the present application. Furthermore, since it is unnecessary to sharpen the cutter tip by grinding its top and only a very small amount of material need be removed from the face during sharpening, the cutting radius of the blade is essentially constant even after repeated sharpening. Additionally, because of the orientation of the cutting tip, the tip must be replaced less often than a tip oriented according to the conventional geometry. Therefore, there is less down time which is highly important in industry where down time is most costly.